The present invention relates to a pneumatic radial tire for a passenger car having a tread-belt structure with two belt layers comprising steel cords, which is improved in driving stability with a reduction in the weight of the tire.
Environmental pollution spreading on an earth scale in recent years has brought about a strong demand for a further lowering in the fuel consumption of vehicles, and, as a part of it, a reduction in the weight of tire has become brought to the fore as a large technical task.
In a pneumatic radial tire for a passenger car, it is known that a high driving stability can be attained when the two belt layers comprise steel cords because the steel cords are far superior in the strength and the modulus of elasticity to other fiber cords. The steel cords, however, unfavorably increase the weight of the tire and the fuel consumption due to their high specific gravity, so that it is difficult for them to cope with the above-described technical task.
It has been believed that, in the pneumatic radial tire having a steel cord belt layer, the total sectioanl area of steel wires per 50 mm width of the steel belt layer must be at least 9.5 mm.sup.2 in order to ensure the driving stability. Reduction in the weight of such a penumatic radial tire may be achievable by decreasing the quantity of wire in the steel cord of belt layer. However, by such a means, the bending rigidity of belt layer decreases, the cornering power drops, and driving stability is deteriorated. Accordingly, it has hitherto been thought practically impossible to reduce the weight of tire while maintaining or improving the excellent driving stability of a tire having a steel cord belt structure.